conf: Search included files under user specified configuration directories

conf: Search included files under user specified configuration directories

Users can include file by giving a relative path or just a file name via
alsaconf syntax <xxx>. ALSA conf will search the file in top configuration
directory and additional configuration directories defined by users via
alsaconf syntax <searchdir:xxx>.

ALSA conf will search and open an included file in the following order
of priority:

1. directly open the file by its name;
2. search for the file name in top config direcotry "/usr/share/alsa/";
3. search for the file name in additional configuration directories
   specified by users, via alsaconf syntax
   <searchdir:relative-path/to/user/share/alsa>;
   These directories will be searched in the same order as their
   definition.

The configuration directories defined by a file will only be used to
search the files included by this file.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>